Optical couplers and hybrids

ABSTRACT

An optical circuit for routing a signal includes a coupler and first and second waveguides. The coupler has an input for the signal and has first and second outputs. The first waveguide has a first optical connection to the first output, and the second waveguide has a second optical connection to the second output. Both waveguides have the same propagation length. The first and second waveguides include different widths at the respective optical connections to the respective outputs. This coupler can be used with another input couplers, two additional waveguides, and two 2×2 output couplers to provide a 90-degree hybrid for mixing signal light and local oscillator light in a coherent receiver or the like.

FIELD OF THE DISCLOSURE

The subject matter of the present disclosure relates to a photonic integrated circuit, and more particularly relates to an optical hybrid formed of an optical coupler network.

BACKGROUND OF THE DISCLOSURE

A coherent receiver can be used in coherent optical transmission and in light detection and ranging (LiDAR) applications. Coherent detection at the receiver typically requires an optical hybrid, such as a 90-degree optical mixer, which mixes signal light (S) with local oscillator light (LO) at the receiver so quadrature phase shift keyed (QPSK) modulation can be used. Highly-Integrated Photonic Integrated Circuits (PICs) are the preferred type of device for these applications.

In general, a 90-degree optical hybrid is a device having two input ports and having four output ports. One input port receives signal light “SIG”, while the other input port receives local oscillator light “LO”. Inside its configuration, the optical hybrid 10 provides a mixture of the signal light (SIG) and the local oscillator (LO) light at each of the four output ports with an optical phase difference incremented by 90-degrees from output to output. When used in a receiver with two pairs of balanced photodetectors (not shown) at the outputs, the receiver can demodulate two quadrature components of an optical QPSK modulation in the received signal light (SIG).

Although optical hybrids of the prior art may be effective for their purposes, an optical hybrid preferably provides a precise phase relationship between its outputs. Phase errors can lower the PIC yield significantly. Therefore, an optical hybrid preferably has minimized phase errors so that the PIC yield can be higher.

The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.

SUMMARY OF THE DISCLOSURE

According to the present disclosure, an optical circuit is used for routing a signal. The optical circuit comprises a coupler and first and second waveguides. The coupler has an input for the signal and has first and second outputs. The first waveguide has a first optical connection to the first output and has a propagation length. The second waveguide has a second optical connection to the second output and has the same propagation length as the first waveguide. However, the first and second optical waveguides comprise different input widths at the respective optical connections to the respective outputs.

According to the present disclosure, an optical circuit is used for mixing first and second signals. The optical circuit comprises input couplers, four waveguides, and output couplers. The input couplers for each of the first and second signals each comprises a first input and two first outputs. The four waveguides each has a first optical connection to one of the first outputs of the input couplers, and each of four waveguides has a same propagation length. The output couplers each comprises two second inputs and two second outputs. Each of the second inputs has a second optical connection to one of the four waveguides. At least two of the waveguides comprise different input widths at the first optical connections to the two first outputs of at least one of the input couplers.

An optical coherent receiver can comprise the optical circuit as described above. Likewise, an optical coherent receiver can comprise two optical circuits as described above, each being connected to one of two polarized signals from two polarization beam splitters.

The foregoing summary is not intended to summarize each potential embodiment or every aspect of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a coupler according to the present disclosure.

FIGS. 2A-2B illustrate different implementations of couplers for the present disclosure.

FIG. 3 illustrates a 90-degree optical hybrid according to the present disclosure.

FIG. 4 illustrates another 90-degree optical hybrid according to the present disclosure.

FIG. 5 illustrates a coherent receiver having 90-degree optical hybrids according to the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure is directed to an optical circuit, where the required phase difference is achieved by waveguides having different widths. Using this configuration in a 90-degree optical hybrid, for example, the fabrication tolerance can be significantly increased.

In one configuration as shown in FIG. 1, an optical circuit 50 of the present disclosure includes a coupler 60, which can be a 1×2 coupler. The coupler 60 includes at least one input 62 to receive a signal from an input waveguide and includes two output 64 a-b. Output waveguides 70 a-b connect at optical connections to the outputs 64 a-b and have different waveguide widths (W₁, W₂). After propagation of the light signal a certain propagation length (L) through the waveguides 70 a-b, a desired phase difference can be achieved between the two outputs 74 a-b of these waveguides 70 a-b. The waveguide widths (W₁, W₂) may be gradually changing during the propagation through 70 a and 70 b.

As will be appreciated, the effective refractive index n_(eff) of the waveguides 70 a-b is a function of their widths (W1, W2). In particular, the effective refractive index n_(eff) for light propagated in the waveguides 70 a-b is analogous to wavenumber (i.e., the change in phase per unit length) caused by a medium. For a given wavelength, the phase constant (β) of the waveguide 70 a-b is given by the effective index n_(eff) times the vacuum wavenumber:

$\beta = {n_{neff}{\frac{2\pi}{\lambda}.}}$

The values of the phase constant (β) are mode-dependent and frequency-dependent and can be calculated with numerical solutions that depend on the refractive index profile of the waveguides 70 a-b.

The effective index n_(eff) depends on the waveguide's cross-section, waveguide materials, and the cladding material. As is known, a mode is characterized by an invariant transversal intensity profile and the effective index n_(eff). Each mode propagates through the waveguides 70 a-b with a phase velocity of c/n_(eff), where c denotes the speed of light in vacuum and n_(eff) is the effective refractive index of that mode. Accordingly, the dimensions of the waveguides 70 a-b determine which modes can exist and the value of n_(eff).

Overall, the effective index n_(eff) of the waveguides 70 a-b depends on the widths of the waveguides 70 a-b and the propagation distance (L). Phase differences accumulate through the waveguides 70 a-b over the propagation distance (L). By configuring the widths (W₁, W₂) of the waveguides 70 a-b and configuring the propagation length (L) afterward, a prescribed phase difference can be achieved at the terminations 74 a-b of the waveguides 70 a-b. Based on the details of an implementation, the component size, the materials, etc., numerical methods can compute the characteristics of the waveguides 70 a-b to achieve this goal of a prescribed phase difference at the terminations 74 a-b.

As will be appreciated, the effective index n_(eff) for the waveguides 70 a-b implemented in silicon photonics is typically determined based on a numerical solution because there is no analytical solution available. Therefore, the widths (W₁, W₂) of the waveguides 70 a-b to achieve the desired phase difference are optimized based on numerical simulations for the implementation at hand. For example, finite-difference time domain (FDTD), beam propagation methods, eigenmode expansion methods, or other approximation techniques can be used to optimize the differences in widths (W₁, W₂) of the waveguides 70 a-b for the particular implementation.

In some implementations, the widths (W₁, W₂) of the waveguides 70 a-b from the two outputs 64 a-b of the coupler 60 may be gradually tapered to the same width (W₀) at the terminations 74 a-b, as shown in FIG. 1. This depends on the additional components to which the circuit 50 is connected. In other cases, the tapering of the waveguides 70 a-b to the same width (W₀) may not be necessary.

As shown in FIG. 1, the coupler 60 can be a 1×2 multi-mode interference (MMI) coupler. Other couplers can be used, such as a Y-junction coupler 61 shown in FIG. 2A. Another coupler that can be used for input includes an adiabatic coupler.

For their part, the waveguides 70 a-b can have any number of available structures, including cores with cladding (e.g., optical fibers) or channel waveguides that are buried, strip-loaded, ridge, rib, diffused, etc. The waveguides 70 a-b can be composed of suitable materials, such as silicon (Si), SOI, InP, InP/InGaAsP, InGaAs—AlGalnAs, LiNbO₃, other semiconductor materials, photonic crystals, etc.

In general, the optical circuit 50 of the present disclosure includes an N×M coupler (60) having N input ports (62) and M output ports (64). Here, N is larger or equal to 1, while M is larger or equal to 2. At least two outputs (64) among the M outputs have different waveguide widths (W₁, W₂). By configuring the widths (W₁, W₂) and the following propagation lengths (L) afterwards, a desired phase shift can be achieved among the different output terminations (74).

As an example, the above coupler 60 in FIG. 1 can be used to construct a 90-degree optical hybrid as shown in the optical circuit 50 of FIG. 3. Here, the optical circuit 50 can achieve the 90-degree phase shift between outputs by using two input couplers 60 a-b and using two output couplers 80 a-b. As diagramed, the input couplers 60 a-b can be 1×2 couplers or splitters, and the output couplers 80 a-b can be 2×2 couplers or combiners. At least one of the two input couplers (e.g., coupler 60 a) has outputs with waveguides 70 a-b of different widths (W₁, W₂) to provide the required phase difference. The other input coupler 60 b may be a phase symmetric optical splitter.

Instead of using at least three 2×2 couplers as used in the prior art configurations, the present optical circuit 50 use two 1×2 input couplers 60 a-b and uses two 2×2 output couplers 80 a-b. The needed 90-degree phase difference can be obtained by properly configuring the waveguide widths (W₁, W₂) after at least one of the 1×2 input couplers (i.e., 60 a). All four of the waveguides 70 a-d connecting the couplers 60 a-b, 80 a-b have the same physical lengths (L).

As noted previously, the input couplers 60 a-b can include an MMI coupler, a Y-junction coupler, or an adiabatic coupler. For their part, the output couplers 80 a-b can include a number of couplers. As shown in FIG. 2B, for example, these output couplers can include a 2×2 MMI coupler (81 a), a 2×2 directional coupler (81 b), a 2×2 adiabatic coupler (81 c). These output couplers can each be a 90-degree optical combiner.

As shown in FIG. 3, the optical circuit 50 has relatively short waveguide interconnects and can operate as a 90-degree optical hybrid. Implemented as a photonic integrated circuit, the optical circuit 50 has the four optical couplers 60 a-b and 80 a-b disposed upon a substrate (not shown) so the couplers 60 a-b, 80 a-b can face inward toward one another.

Each of the inner ends of the four couplers 60 a-b, 80 a-b has two optical outputs 64 a-d, 84 a-d interconnected with the four optical waveguides 70 a-d in a compact arrangement. The opposing input couplers 60 a-b respectively split an input 62 a-b into two outputs 64 a-b, 64 c-d. The opposing output couplers 80 a-b have two inputs 82 a-b, 82 c-d respectively and have two outputs 84 a-b, 84 c-d respectively. Each waveguide 70 a-d connects one of the output 64 a-d of the input couplers 60 a-b to the input 82 a-d of an adjacent output coupler 80 a-b so that the waveguides 70 a-d are arranged a non-intersecting manner without waveguide crossings.

As disclosed herein, the optical circuit 50 can be used as an optical mixer, wherein light received by the two input couplers 60 a-b is mixed by the circuit 50 and exits from the outputs 84 a-d on the output couplers 80 a-b. During operation, for example, an optical signal (SIG) is provided at the input 62 a of a first input coupler 60 a via a first input waveguide, while a reference optical signal, such as LO light, is provided at the input 62 b of a second input coupler 60 b via a second input waveguide. Of course, the signal SIG light and the LO light may be switched.

The input coupler 60 a outputs the signal light SIG from the outputs 64 a-b, and the other input coupler 60 b outputs the LO light from the outputs 64 c-d. The waveguides 70 a-b from the signal's outputs 64 a-b having the same physical length (L) (but different widths) guide the signal (SIG) light from the output 64 a-b into the adjacent inputs 82 a, 82 c of each of the output couplers 80 a-b at a desired phase difference (i.e., 90-degrees). Each of the output couplers 80 a-b then split the signal (SIG) light into two preferably equal portions, and couple them into the respective two outputs 84 a-d of the corresponding couplers 80 a-b adding a 90-degree phase shift therebetween.

Concurrently, the other optical waveguides 70 c-d from the LO's outputs 64 c-d having the same physical length (L) (and same width) guide the LO light from the outputs 64 c-d into the adjacent inputs 82 b, 82 d of each of the output couplers 80 a-b. Each of the couplers 80 a-b then split the LO light into two preferably equal portions, and couple them into the respective two outputs 84 a-d of the corresponding coupler 80 a-b with the added 90-degree phase shift therebetween.

As noted, at least one of these input couplers (e.g., 60 a) can be phase symmetric so that the outputs have substantially the same phase, possibly subject to some small phase errors in the coupler. However, as noted, at least one of the input couplers (60 a) can output to waveguides 70 a-d of different widths (W₁, W₂) so that the output light has a 90-degree phase shift therebetween after traversing the length (L) of the respective waveguides.

In FIG. 3, the first input coupler 60 a for the signal light (SIG) has outputs 64 a-b to waveguides 70 a-b of different widths (W₁, W₂) according to the present disclosure. Other configurations can be used. For example, the second input coupler 60 b for the LO light (LO) may instead have this configuration, or both of the input couplers 60 a-b may have these configurations, as long as desired phase differences can be achieved at the outputs of the circuit 50.

As a result, the four outputs 84 a-d of the output couplers 80 a-b output mixed signal SIG light and LO light at incremented phase shifts of 90-degree from port to port. Additional waveguides can then guide the mixed signal SIG light and LO light with incremented 90-degree phase shifts from the respective output ports 84 a-d to other optical elements for processing, which can be arranged at the two opposite edges of a chip on which the optical circuit 50 is formed. These other optical elements can include those found in a coherent receiver. In the coherent receiver, for example, waveguides at the inputs 62 a-b can be connected to optical fibers, and waveguides at the outputs 84 a-d can be coupled to photodiodes via lensing.

As shown in FIG. 3, the optical circuit 50 has the couplers 60 a-b, 80 a-b arranged along two orthogonal axes in a cross-like arrangement. This represents a compact configuration that advantageously allows each of the interconnects for the waveguides 70 a-d to be of equal length (L) and each at a 90-degree waveguide bend. A radius of the waveguide bend defines the allowable distance between the ends of the opposing couplers 60 a-b, 80 a-b and can be restricted by limitations on radiative loss in the waveguide bends.

Although this cross-configuration is used in FIG. 3, other configurations could be used. For example, FIG. 4 illustrates another optical hybrid 50 according to the present disclosure in another configuration. Here, the waveguides 70 a-b from one input coupler 60 a use S-bends to connect to respective inputs 82 a, 82 c of output couplers 80 a-b. The other input coupler 60 b uses 180-degree waveguides 70 c-d to connect to respective inputs 82 b, 82 d of the output couplers 80 a-b. Details concerning the widths (W₁, W₂) and lengths (L) of the waveguides 70 a-d for at least one of the input couplers (e.g., 60 a) are the same as disclosed previously. This compact arrangement may be suitable for a given implementation, depending on the device in which the hybrid 50 is used, the amount of space available, etc.

As noted, the optical circuits 50 of the present disclosure may be implemented in a chip as a photonic integrated circuit (PIC) device. The optical circuits 50 can have high fabrication tolerances because the connecting waveguides 70 a-d have the same physical lengths (L) and can be very short. In addition, the input couplers 60 a-b can have lower insertion loss and broader bandwidth than the output couplers 80 a-b, and hence more fabrication tolerance.

The optical circuits 50 of the present disclosure can be used in a number of different implementations and devices. As one example, FIG. 5 diagrams the structure of a coherent receiver 100, which can use optical circuit 50 a-b for the 90-degree optical hybrid disclosed herein.

The local oscillator LO signal can be provided by an external cavity laser 102 or the like. The signal (SIG) light and LO light pass through polarization beam splitters 110 a-b. Horizontally polarized light passes to one of the 90-degree optical hybrids 50 a, while vertically polarized light passes to the other hybrid 50 b. Internally, the hybrids 50 a-b shift the phase of the SIG light and LO light as noted above. For each hybrid 50 a-b, first outputs from one of the output couplers (80 a) are optically imaged on first balanced detectors 120, while second outputs for the other output couplers (80 b) are optically imaged on other balanced detectors 120. The balanced detectors 120 connect to transimpedance amplifiers 130 and analog-to-digital converters 140, which provide the signals to a digital signal processor DSP circuitry 150 for processing based on quadrature phase shift keyed (QPSK) modulation or some other in-phase/quadrature modulation used.

The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter. 

What is claimed is:
 1. An optical circuit for routing a signal, the optical circuit comprising: a coupler having an input for the signal and having first and second outputs, the coupler being configured to output the signal at the first and second outputs; a first waveguide having a first optical connection to the first output and having a propagation length; and a second waveguide having a second optical connection to the second output and having the propagation length, wherein the first and second optical waveguides comprise different input widths at the respective optical connections to the respective outputs, wherein the first and second waveguides comprise a same output width at terminations of the propagation length, and wherein the different input widths between the respective first and second waveguides at the respective optical connections are configured to produce a phase difference in the signal along the propagation length to the terminations.
 2. The optical circuit of claim 1, wherein the coupler comprises a 1×2 multi-mode interference coupler, a Y-junction coupler, or a 1×2 adiabatic coupler.
 3. The optical circuit of claim 1, wherein an intermediate width of at least one of the first and second optical waveguides changes gradually along the propagation length from the input width to an output width.
 4. The optical circuit of claim 1, wherein the different input widths at the respective optical connections are configured to produce the phase difference of 90-degrees in the signal along the propagation length between the respective first and second waveguides.
 5. The optical circuit of claim 1, comprising more than one of the input.
 6. The optical circuit of claim 1, comprising more than the first and second outputs.
 7. An optical circuit for mixing first and second signals, the optical circuit comprising: input couplers for each of the first and second signals, each input coupler comprising a first input and two first outputs, the input couplers being configured to output the respective first and second signal at the first outputs; four waveguides each having a first optical connection to one of the first outputs of the input couplers, each of the four waveguides having a same propagation length; and output couplers each comprising two second inputs and two second outputs, each of the second inputs having a second optical connection to one of the four waveguides, wherein at least two of the waveguides comprise different input widths at the first optical connections to the two first outputs of at least one of the input couplers, wherein the at least two of the waveguides comprise a same output width at terminations of the propagation length, and wherein the different input widths between the at least two of the waveguides at the respective optical connections are configured to produce a phase difference in the respective first and second signal along the propagation length to the terminations.
 8. The optical circuit of claim 7, wherein a first pair of the four waveguides for both of first and second of the input couplers connect to a first of the output couplers; and wherein a second pair of the four optical waveguides from both of the first and second input couplers connect to a second of the output couplers.
 9. The optical circuit of claim 8, wherein first and second of the input couplers are disposed opposite one another; wherein the first and second output couplers are disposed opposite one another offset from the first and second input couplers; and wherein the four optical waveguides comprise 90-degree bends arranged in a non-intersecting manner.
 10. The optical circuit of claim 8, wherein first and second of the input couplers are disposed opposite one another; wherein the first and second output couplers are disposed on either side of the second input coupler; wherein the first pair of the four waveguides from the first input coupler comprise S-bends each connected to one of the second inputs of the first and second output couplers; and wherein the second pair of the four waveguides from the second input coupler comprise 180-degree bends each connected to another of the second inputs of the first and second output couplers.
 11. The optical circuit of claim 7, wherein the second outputs are configured as a 90-degree optical hybrid.
 12. The optical circuit of claim 7, wherein the input couplers each comprises a 1×2 multi-mode interference coupler, a Y-junction coupler, or a 1×2 adiabatic coupler.
 13. The optical circuit of claim 7, wherein the output couplers each comprises a 2×2 multi-mode interference coupler, a 2×2 directional coupler, or a 2×2 adiabatic coupler.
 14. The output circuit of claim 7, wherein the output couplers each comprises a 90-degree optical combiner.
 15. The optical circuit of claim 7, wherein at least one of the input couplers comprises a phase symmetric optical splitter.
 16. The optical circuit of claim 7, wherein an intermediate width of at least one of the at least two waveguides with the different input widths changes gradually along the propagation length from the input width to an output width.
 17. An optical coherent receiver comprising the optical circuit of claim
 7. 18. An optical coherent receiver comprising two optical circuits of claim 7, each connected to one of two polarized signals from two polarization beam splitters. 